(CNN) - Deeming congressional rejection of his border national emergency "reckless" and "dangerous", President Donald Trump issued the first veto of his presidency Friday, insisting the situation on the southern frontier amounted to a threat to Americans' safety.

"Congress has the freedom to pass this resolution and I have the duty to veto it," Trump said from the Oval Office before officially sending the measure back to Congress without his approval.

It is the first time in his two years in office that Trump has used his presidential veto power to block legislation and comes after a dozen Senate Republicans joined Democrats to rebuke Trump's use of his national emergency power to bypass Congress and fund construction of a border wall.

Trump said the resolution, which would have reversed the national emergency, "put countless Americans in danger."

"There haven't been too many that are bigger emergency than what we have" at our border, he said.

Trump was surrounded at Friday's event by officials from Customs and Border Protection as well as surviving family members of those who have loved ones killed by undocumented immigrants. Attorney General William Barr was also at the President's veto event.

While some lawmakers -- including some Republicans -- have argued against the President's use of national emergency powers in this instance, the Justice Department set forth a robust defense of the President's authority to do so in a letter to Senate Majority Leader Mitch McConnell earlier this month, according to a copy obtained by CNN on Friday.

"The President acted well within his discretion in declaring a national emergency concerning the southern border," wrote Assistant Attorney General Stephen Boyd, setting out the legal basis for the proclamation under the National Emergencies Act and additional statutory authorities, which largely tracks an internal memo issued by the Office of Legal Counsel at the Justice Department.

"The President's emergency Proclamation reasonably described the current situation as an ongoing 'border security and humanitarian crisis,'" Boyd adds. "The crisis at the border ... may qualify as an emergency even though it, too, is not entirely new."

Twelve Republican senators banded together Thursday to deliver the forceful rebuke after expressing concerns that Trump's use of the national emergency declaration as an end-run around Congress violates the separation of powers and sets a bad precedent that a would-be future Democratic president could follow to unilaterally drive their agenda.

The White House sought to pare back Republican defections leading up to the vote, with the President and White House aides making clear to Republican senators that a vote against Trump on this issue would have ramifications come re-election time.

Trump rejected entreaties from several Senate Republicans to agree to a compromise that would curtail his national emergency powers and instead framed the vote not as a matter of constitutional concerns, but rather as a litmus test on border security.

The approach -- particularly the threats of re-election repercussions -- stemmed defections from several Republicans up for re-election in 2020, but ultimately failed to stop the Senate from passing the resolution.

Trump tweeted about the political advantage he expects those who supported him will receive.

"I'd like to thank all of the Great Republican Senators who bravely voted for Strong Border Security and the WALL. This will help stop Crime, Human Trafficking, and Drugs entering our Country. Watch, when you get back to your State, they will LOVE you more than ever before!" Trump tweeted Friday.

Trump's veto sends the resolution back to the US House of Representatives, which is expected to pick it up after the week-long congressional recess. The House is not expected to have the two-thirds of the chamber's support needed to override the President's veto.